1. Field of the Invention
The present invention relates generally to wireless communications, and more particularly, to a channel state information request/feedback method and apparatus for a wireless communication system supporting carrier aggregation or bandwidth extension.
2. Description of the Related Art
Long Term Evolution-Advanced (LTE-A) is standardized by the 3rd Generation Partnership Project (3GPP). In the 3GPP, a Bandwidth Extension technique is discussed to support a peak data rate that is comparatively higher than the LTE Release 8.
The Bandwidth Extension, based on the Carrier Aggregation technique, can extend the bandwidth beyond a single LTE Release 8 system bandwidth. In the Bandwidth Extension, each bandwidth for transmission is called a Component Carrier (CC).
A Release 8 LTE User Equipment (UE) is specified to use a single Component Carrier in UpLink (UL) and DownLink (DL), respectively.
The Component Carriers (CCs) can be aggregated contiguously or non-contiguously in a frequency domain.
FIG. 1 is a diagram illustrating a relationship of a DL CC 100 and a UL CC 101 in the LTE Release 8 system.
In LTE Release 8, each UE performs communication with the single DL CC 100 and the single UL CC 101. The UL grant and DL assignment are transmitted on the DL CC 100, and the PUSCH corresponding to the UL grant is transmitted on the UL CC 101 associated with the DL CC 100. In FIG. 1, the solid line connecting the DL CC 100 and the UL CC 101 shows the relationship between the DL CC 100 and the UL CC 101 on which the PUSCH transmission is scheduled by means of the UL grant transmitted on the DL CC 100.
When data to be transmitted occurs, the LTE Release 8 UE receives scheduling information for transmitting a Physical Uplink Shared Channel (PUSCH) in a Physical Downlink Control Channel (PDCCH) transmitted by a base station (eNodeB or eNB). The scheduling information can be called an UL grant. In LTE Release 8, only one Downlink Control Information (DCI) format, i.e. DCI format 0, is defined for the UL grant. In order to check the channel state of the downlink carrier on which the PDCCH is transmitted, the eNB requests the UE for aperiodic Channel State Information (CSI) with a 1-bit CSI request field of DCI format 0. When the signal of DCI format 0 is received, the UE analyzes the CSI request field contained in the received signal, measures channel state in the subframe in which the signal is received, and feeds back the measured aperiodic CSI of the DL CC 100 to the eNB. The aperiodic CSI is multiplexed with the data to be transmitted by the UE on the PUSCH. Although the aperiodic CSI is multiplexed with the data to be transmitted by the UE on the PUSCH, it is simply referred to as “transmits aperiodic CSI” herein.
In LTE-A, a UE can communicate multiple downlink Carrier Components and/or multiple uplink Carrier Components with the bandwidth extension technique. When using carrier aggregation, however, the conventional method has is problematic when the eNB requests the aperiodic CSI and the UE feeds back the requested aperiodic CSI.